The clinical utility of peritoneovenous shunt patency studies using intraperitoneal Tc-99m macroaggregated albumin was studied. The patency of a peritoneovenous shunt can be reliably determined using the intraperitoneal injection of Tc-99m macroaggregated albumin with the appearance of lung activity as an indicator of patency. In an evaluation of 66 cases of clinically suspected peritoneovenous shunt obstruction, using the appearance of lung activity as the sole criterion of patency, this technique was shown to have a sensitivity of 100%, specificity of 92.2%, and accuracy of 98.5%. Visualization of efferent shunt tubing was variable and not a reliable criterion of shunt patency. This technique should be considered when peritoneovenous shunt patency is questioned.